Advanced electrochemical performance of Li[Ni(1/3−x)FexCo1/3Mn1/3]O2 as cathode materials for lithium-ion battery

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Abstract

Li[Ni(1/3−x)FexCo1/3Mn1/3]O2 (x=0, 0.05) cathode materials with α-NaFeO2 structure were synthesized by co-precipitation method. Hexagonal a and c lattice parameters increased with Fe-dopant. The heat treatment process of precursor were investigated by TG-DTA, the structures of samples were confirmed by X-ray diffraction (XRD) and Fourier Transform Infrared spectra (FT-IR), the distribution of narrow particle size and morphologies were observed by scanning electron microscopy (SEM) and the charge and discharge capacity and electrochemical cycle stability were tested by Battery Testing System (BTS). In addition, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the reaction of lithium ion insertion and extraction from materials. The results indicated that the structure of dopant samples was more stable than that of the native and the discharge capacity and the electrochemical cycle stability were also improved by doping Fe.

Introduction

At present, LiCoO2 is the most widely used cathode electrode material in commercial lithium ion batteries, as it is easy to prepare and has good rechargeability even at high rate performances [1]. However, the toxicity and high cost of cobalt represent some problems for this material. Many researchers have tried to improve reversible capacity, cycle life and thermal stability of this materials by using Co–Ni [2], Mn–Co [3], [4] or Mn–Ni [5] mixed oxides. But these do not satisfy all the needs of commercial lithium ion batteries. This issue has attracted the interest of various researchers ever since Ohzuku’s early work of Li[Ni1/3Co1/3Mn1/3]O2 in 2001 [6]. There are many attractive characteristics for the combination of nickel, manganese and cobalt, such as higher theoretical capacity (280 mAh g−1), lower cost, and less toxicity [6], [7], [8], [9]. Despite the material’s advantages, it is considered to be one of the best candidates as cathode electrode material for hybrid electric vehicle (HEV) power source system by Amine and co-workers [10]. What is more, Dahn and co-workers reported that larger particles having higher tap-density would be less reactive at highly oxidized state.

Earlier studies show that it is difficult to prepare Li[Ni1/3Co1/3Mn1/3]O2 by using traditional solid-state for the presence of undesired impurities such as metal oxide in the final sample. This material has inferior electrochemical performance due to the resulted inhomogeneous phase [11], [12], [13], [14]. On the other hand, the crystallinity, phase purity, particle morphology, grain size, surface area, and cation distribution in the structure, all play important roles in the electrochemical performance of the compound, though they are strongly determined by the synthesis procedure [15], [16]. In order to synthesize uniform and homogeneous Li[Ni1/3Co1/3Mn1/3]O2, the selected synthetic method is important to obtain phase-pure final product. Co-precipitation method is the best way to prepare uniform and homogeneous metal hydroxide, such as triple or quadruple hydroxide. Also it is easy to get spherical particles and high tap-density samples by this method [17].

In this study, the authors substituted a small amount of Ni in Li[Ni1/3Co1/3Mn1/3]O2 with Fe, such as Li[(Ni0.95Fe0.05)1/3Co1/3Mn1/3]O2. The crystal structures and electrochemical properties of these compounds are also investigated in this paper.

Section snippets

Materials preparation

[Ni(1/3−x)FexCo1/3Mn1/3](OH)2 (x=0, 0.05) were prepared as follows: An aqueous solution of NiSO4, CoSO4, MnSO4 and FeSO4 with a concentration of 2.0 mol dm−3 was added to strongly stirred tank reactor under argon atmosphere. At the same time, NaOH solution of 2.0 mol dm−3 and some NH3⋅H2O solution (28%–30%) as a chelating agent were also added into the reactor. The pH of preliminary solution was always 11 by adding NaOH solution during co-precipitation reaction. [Ni(1/3−x)FexCo1/3Mn1/3](OH)2 (x=

Materials synthesis and characterization

TG/DTA curves for mixing [(Ni0.95Fe0.05)1/3Co1/3Mn1/3](OH)2 and a amount of LiOH⋅6H2O (Li:[Ni+Co+Mn+Fe]=1.05:1) are shown in Fig. 1. As can be seen, there are three peaks on the DTA curve which are at the peak temperatures of 80 C,260 C and 485 C, respectively. Meanwhile, three weight loss stages are clearly observed on the TG curve. The first endothermic peak that occurs at 80 C is attributed to the evaporation of water molecules absorbed by samples. The second exothermic peak that happens

Conclusion

The layered Li[Ni(1/3−x)FexCo1/3Mn1/3]O2 (x=0,0.05) cathode materials were successfully synthesized by co-precipitation method. The effect of Fe substitution on the structure, morphology and electrochemical properties of Li[Ni(1/3−x)FexCo1/3Mn1/3]O2 was investigated. The two samples have a phase-pure hexagonal α-NaFeO2 structure with a space group of R3¯m with no obvious impurity phase peaks. After doping Fe, the lattice parameters both in the a- and c-directions become large and the particle

Acknowledgements

This work was supported by the Nature Science Foundation of China (20571019). The Project was sponsored by SRF for ROCS, HLJ(LC06C13) and by Program of Harbin Subject of Chief Scientist.

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